1. Field of the Invention
This invention relates to a method of removing coating films, more particularly, to a method by which coating films on resin substrates are first treated with alkalies to be rendered easily strippable and then subjected to specified polishing treatments so as to remove the coating films from the surfaces of the resin substrates, whereby they are reverted to resin materials that can be recycled to industrial uses including the one to which they were initially put to.
2. Discussion of Related Art
Compared to metal and glass products, shaped resin parts are not only lightweight and superior in impact resistance but also inexpensive and easy to work by shaping. Because of these advantages shaped resin parts are currently used in automobiles, home electric appliances, daily sundries and many other areas. In particular, for the purpose of reducing the body weights, the number of resinous parts used in automotive bodies has been increasing.
Most of such resin products are coated in order to improve not only their resistance to bruise and weather but also their aesthetic appearance. To insure strong adhesion to the shaped resin parts and to produce coating films of high performance, the paints for coating are mostly made of materials capable of forming three-dimensional crosslinked structures. However, because of this excellent coating performance, the films having strong crosslinked structures cannot be easily stripped even if there is some reason to strip them after once formed by coating.
The removal of coating films is industrially an important technology since it is required on many occasions such as where defects have occurred in the coating process, where the old coating film with deteriorated performance has to be replaced by a new one and where there is a need to reuse only the resin substrates of the coated shaped resin parts. The heretofore practiced methods of stripping and removing coating films are roughly divided into physical and chemical means. The physical stripping is a method in which the coating films are scraped off with sand paper or the like. The chemical stripping is a generally classified into the following three methods:
1) removal with strippers that are based on chlorine-containing solvents (Japanese Patent Publication (kokoku) Nos. Sho 51-34238, 57-76065 and 59-117567); PA1 2) removal with strippers that are based on organic materials (Japanese Patent Application (kokai) Nos. Sho 50-109925, Hei 1-289878 and Hei 2-274775); and PA1 3) removal with strippers that are based on inorganic materials (Japanese Patent Application (kokai) Nos. Sho 50-109925, 59-131674 and 61-162568).
In other methods, the coating films are removed by means of strippers in which the main components of the three strippers mentioned above are mixed in suitable proportions either with themselves or with other components.
However, depending on the kind, thickness and other factors of the coating film to be removed, the above-mentioned prior art methods have sometimes failed to achieve complete removal of the coating films. In certain cases, there is a need to recycle the resin product to the same use after it has been stripped of the coating film but, if its removal is incomplete, the physical properties and the like of the product will deteriorate.